Nematodes: the almost invisible crop yield thief

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“We’ve got a lot of nematodes in our soybean fields,” says Tom Allen, assistant Extension and research professor at the Delta Research and Extension Center, Stoneville, Miss., who spoke at the annual meeting of the Mississippi Agricultural Consultants Association. “And unlike the Midwest, which has predominantly soybean cyst nematodes, we have several species in Mississippi that can cause yield reductions.”

Southern nematode survey

He is participating in a United Soybean Board-funded project across the southern U.S. to determine the important nematode species and infestation levels. In 2011, each state submitted samples for analysis to determine the most significant nematode problems in the respective state. Mississippi submitted 77 samples from 32 counties.

As expected, the analyses indicated that reniform was the predominant nematode pest throughout Mississippi. Other nematodes included lesion, spiral, root knot, and to a lesser extent, soybean cyst nematode.

“A lot of cotton fields that have been moved into soybean production in recent years have a tremendous reniform problem,” Allen says. “Four of the 77 samples were above the threshold that we would expect yield loss. Two fields were above the threshold where we would expect a yield loss. However, another 11 fields had moderate reniform levels. One field was above the threshold for root knot nematodes; however two fields had extreme root galling present on soybean plants in the field.”

Samples from four fields in the northwestern corner of the state had some soybean cyst nematodes present, he says. “In one field, a non-host crop had been grown since 2007 and the grower still had detectable SCN.”

If a SCN-infested field also has sudden death syndrome, the combination “can cause even more severe yield losses,” he says. “If you have SDS, please take a soil sample and send it in so we determine if there is also a SCN problem.”

In the area of soybean diseases, 2011 saw a significant aerial web blight problem in eastern and northeastern Mississippi.

“Aerial blight can decimate an entire soybean field if you’re not paying attention,” Allen says. “It will knock off blooms and pods, as well as leaves. You can put a combine in the field at the end of the season and cut nothing.”

High temperatures in 2010 and 2011 combined with sporadic rainfall and high humidity did “a really good job of cooking up aerial web blight. Couple those environmental conditions with the type of fields in the hills region that have trees bordering them and limited air flow, and it only adds to the problem. For scouting purposes, you have to get into the field and part the canopy back to find aerial blight. It can be sporadic across a field where some localized areas have more disease than others.”

Allen said he and retired Extension plant pathologist Billy Moore have conducted a number of fungicide trials, and “the take-home message is: If you have an aerial web blight problem, choose a strobilurin-based fungicide and apply it.

“Economically speaking, one application of a strobilurin-based fungicide made in the presence of the disease at R-5 or a bit earlier, will pay for itself. Use the labeled rate of the fungicide — don’t mess with this disease.”

In research plots, Allen says, “We easily saw a significant difference between treated and untreated areas.”

Aerial application is not advisable, he says. “You need to apply the fungicide with a ground rig, using as much water pressure and as much water volume as possible. I would prefer 60 psi or more and 20 gallons of water, with a full rate of the labeled fungicide.”